And I'll huff and I'll puff and I'll blow your house down
Welcome to Minnesota's summer. A time for storms. When we developed our hazard tree guidelines back in 1989, we felt we had a good understanding of tree defects and tree failures. Since that time, we have had to opportunity to visit four separate catastrophic storm events where trees have been severely damaged: tree failures a' la carte. The most recent storm "event" occurred during the evening of July 1st across the central part of the state cutting a wide swath that included Monticello east through Sherburne County, western Anoka County, and on into Wisconsin. This was the fourth "autopsy" since the hazard tree guidelines came into existence so we couldn't resist touring some of the hardest hit areas and assessing the damage with the guidelines firmly in mind. In particular, the questions asked were "What failed and why did it fail?" We tallied data on 187 trees. Here are some of the data and some conclusions.
Trees did not fail at random, but failed in regular, almost uniform, patterns. The number of trucks driving by us that day with silver maple branches and leaves hanging out prompted our assistant for the day, a forestry "intern", to comment that she would have a hard time ever looking at silver maple into future with anything other than a seriously skeptical eye. Our survey was not statistically designed and suffers from several fatal flaws in that sense. We only looked at what had failed, we did not evaluate the survivors. Ah, the survey of the next storm!
One, we went into areas with serious damage, where trees had been pushed to and beyond their mechanical and structural capacities.
Two, we only recorded data on trees that had failed in a major way. We did not record information for trees in the same areas that had NOT failed.
Three, the areas we visited are typical of urban areas on the Anoka Sand Plain. The species composition is heavy to the urban intrusions of green ash, silver maple, and blue spruce and the native remnant of oak, largely northern pin with greatly reduced numbers of burr and red oaks. Clearly these trees failed most commonly because they are most common. Others failed less often because they are less common. The pattern of failure is, however, unmistakable. We have seen it before, we will see it again.
Trees failed for one of two reasons. First, they failed because they had defects. Namely, "visible defects" that would have been detected in a basic hazard tree evaluation. Specifically, silver maple's failed due to weak unions with included bark (70%) and broken branches with decay (30%). This is a typical pattern with silver maple in storms. Northern pin oak failed due to the presence of decay columns in the main stem and branches. These were commonly complicated by old codominant stems with weak unions, included bark, and subsequent main stem decay in the "hot zone". (Yes, Virginia, there really is a "hot zone".)
The second mode of failure was a fracture of the root plane causing the tree to tip over with root/soil mound that rarely exceeded four feet from side to side. These trees were clearly improperly anchored. They simply did not have enough root mass to support the stem. Two species tipped over with a startling regularity: green ash and blue spruce. Arguably the two most commonly planted trees in the urban landscape, these two trees typically do not establish sufficient root structure relative to the tops they grow. Nicknamed the sail effect for good reason we found these two species failed with a pattern almost frightening in its regularity. The other noteworthy aspect was that these trees did NOT exhibit external, visible defects that would have been obvious during a hazard tree inspection.
In detail, we observed 18 green ash and 21 blue spruce that had failed. Of the Green ash, more than half (10/18) failed for visible cause, ie. Included bark with codominant stems. Blue spruce failed due to windthrow, ONE HUNDRED PERCENT! Both of these species exhibited a major portion of root plate failures on otherwise "defect-free" trees. It is worthy to note that only 3 of 79 oaks that failed did so as root plate failures and ALL THREE had root rot caused by construction damage. In addition, sugar maple (2/2), silver poplar (5/6), and black cherry (2/3) exhibited a high proportion of root plate failures.
Two additional observations bear repeating. Blue spruce is quite windfirm until it gets old enough to be tall enough to get 10-15' of crown above the general line of the roof tops in the neighborhood. This factor alone may help explain why large, old blue spruce are hard to come by except in older, better canopied neighborhoods with lots of two-storied houses. And here you thought they died from Cytospora canker or needle cast disease. How does that commercial go? No.o.o.o.t exactly!
The other observation is the generally small size of the root/soil mass raised when green ash failed. These were quite consistent in size and commonly were reminiscent of the size of balled and burlaped root balls on transplanted trees. I can't help but wonder if this species is having a problem reestablishing the necessary five to seven large, woody, primary lateral roots after transplantation into the well watered, well watered urban landscape. After seeing this pattern for four wind events, I can't help but wonder if we are not growing these two species wrong: too much water, too much fertilizer, too fast of growth, too much top, TOO LITTLE root.
Thinking back to my own house, I'm having some serious problems with that blue spruce of my neighbor's, fast growing, ten feet from the lot line, three feet short of the roof line, immediately west of my kitchen. What's wrong here? My assistant went home that night a little worried about the silver maple just west of her folk's home, too. I commonly hear people talk about "acts of God" and "random thunderstorms", and tales of the tree that just missed the house or car in the driveway. We can and should do better, much better!